The invention relates to a television camera, and more particularly to a color television camera employing a solid state image sensor.
A conventional color television camera includes a single or three pickup tubes, on the photocathode of which is focussed an image which is continuously scanned in both horizontal and vertical directions by a single electron beam in order to derive a video signal. The geometrical distortion of the resulting image is substantially determined by the linearity with which the electron beam scans. Where a plurality of pickup tubes are used, the respective electron beams must be synchronized in time and aligned in geometrical position, which requires a high level of skill. With a color television camera including three pickup tubes, an image information is separated into three color components by means of dichroic mirrors before it is converted into three color components. In this instance, the registration between the colors presents a significant problem. With a color television camera employing a pair of pickup tubes, one of the tubes is used for the brightness signal while the other is used for the color signals. For example, a stripe filter is used to derive red and blue signals, while the green signal is derived from the brightness signal and the red and blue signals by means of a matrix circuit. With a color television camera employing a single pickup tube, a complex stripe filter is used to derive the brightness signal and the color signals. It is well known that the color signals can be derived by the use of a stripe filter according to a phase or frequency separation technique incorporating a 1H delay circuit, where H represents the period of a horizontal scanning frequency. In a single tube color camera incorporating a stripe filter, the problem of the registration is avoided even though there may be problems associated with the image quality and interference patterns and moire resulting from the use of stripe filter. A stripe filter often comprises a combination of vertical or oblique stripes. A stripe filter comprising horizontal stripes has been proposed, but has not been practically embodied because of the technical difficulty of reliably scanning the electron beam along a thin horizontal stripe.
On the other hand, in the field of a black-and-white television camera, the use of a solid state image sensor utilizing semiconductor technology such as a recently developed charge-coupled device (CCD) or a charge transfer device in place of a pickup tube has been proposed. Such solid state image sensor or a black-and-white television camera using it are described in detail in "Imaging With Charge Transfer Devices", Session 2 of 1974 IEEE Intercon Technical Papers. Briefly, taking CCD which is one type of the solid state image sensor, it comprises a plurality of MOS (metal-oxide film-semiconductor) capacitors comprising semiconductor depletion layers which are arranged in a regular array of rows and columns. An image is optically focussed onto the photoelements of the surface to cause the individual depletion layers to store an electric charge which depends on the intensity of incident light thereon. The charge is sequentially transferred by a set of externally applied clock pulses or driving pulses having the same period, thereby reading out the image information as a video signal. Representing the time required for the stored charge to be transferred from one depletion layer to another by the clock pulse by tc, it will be appreciated that tc is equal to the period of the clock pulse. Thus, the transfer time tc can be chosen arbitrarily by changing the frequency of the clock pulse. Assuming that a horizontal row includes N electrodes for the respective depletion layers contained therein, it is seen that the time T.sub.D required to read out the charges of the row by transfer is given by: EQU T.sub.D = N .times. tc
The value of N and tc can be chosen so that T.sub.D becomes equal to one horizontal scanning period of the standard television system, and the various parameters can be chosen such that the completion of transfer of one horizontal row is followed by the transfer of charge along a vertically adjacent next horizontal row so that the transfer in the vertical direction is completed within a period of time which is equal to one vertical scanning period of the standard television system, thus enabling such solid state image sensor to be utilized in place of a conventional pickup tube. As a matter of practice, the determination of N, tc and other parameters must take into consideration the retrace time of the horizontal and vertical scanning lines. Such a solid state image sensor has a number of advantages including its compactness, light weight, low power dissipation and high reliability. Another significant feature as compared with a conventional pickup tube is its extremely small geometrical distortion of the image, as will be appreciated from its arrangement and principle of operation.